Gas turbine engine generating capacities continue to increase, and combined cycle output for a single engine now exceeds 500 MW. Higher power output machines tend to be physically larger, and one power limiting characteristic is the size of the last row of the rotating turbine blades, since the centrifugal force generated in such long blades can exceed the material strength capability of known alloys.
Several techniques have been developed to reduce the weight of turbine blades, thereby facilitating the design of ever larger machines. U.S. Pat. No. 5,626,462 to Jackson et al. discloses a double walled airfoil where an outer skin is metallurgically bonded to an inner support wall. The double wall contains integral cooling channels. However, the bonding of the outer skin and the inner support wall and sharp corners created at the bonds allow for stress risers that may affect component life. U.S. Pat. No. 8,079,821 to Campbell et al. discloses inner and outer walls connected by a compliant structure to enable thermal expansion between the inner and outer layers. However, this arrangement may require complex manufacturing steps to secure the compliant members to the inner and outer walls. U.S. Pat. No. 8,720,526 to Campbell et al. discloses a process for forming a long gas turbine engine blade having a main wall with a thin portion near a tip. In Campbell, a blade is cast having a tip that is thicker than desired. The tip is subsequently machined to the desired size, which adds cost to the manufacturing process. U.S. Pat. No. 8,979,498 to Mazzola et al. discloses creating an airfoil by attaching a cast tip to a cast base via metallurgical bonding or fasteners. However, because it is cast, the tip is limited to characteristics achievable via the casting process.
As the next generation of even larger gas turbine engines is demanded in the marketplace, further improvements in blade design and fabrication will be required.